For a petrol engine it is common to use the negative terminal on the coil as a tach input signal. This signal (decaying sinus) peeks at +/- 200V for every ignition pulse (2 times per RPM for a 4 cylinder 4 stroke engine, 3 for a 6 cylinder etc.). For this type of input, a single rectifier diode plus a peek detector (could be a simple voltage divider) wired to a digital input pin works well.

For a diesel engine it is common to use the W output from the alternator as a tach sensor. This is a pulse train at approximate alternator output voltage (e.g. in the 13.7V to 14.4V range against Gnd). The pulse rate is proportinal to engine RPM, but typically not in a 1:1 ratio so calibration is needed. I would wire this input directly to an Arduino digital pin through a large series resistor (e.g. 100k) and then count pulses using interrupts. You would need another tach to figure out the calibration constant needed for your RPM calculation unless you can get documentatioin on the alternator RPM to engine RPM ratio from the car manufacturer.